Some Fine Doubles
The two measures of a double star are separation and position angle. The directions shown here are for an inverting scope, such as a Newtonian reflector. Other telescopes may have different field orientations.
Sky & Telescope illustration.
I often begin my observing evenings with the famous Double-Double, Epsilon (e) Lyrae.
It's a handy check for seeing conditions and a stunning pair of pairs. Some observers think of Epsilon Lyrae as a tough test of telescope quality, but I've found that most scopes will split both pairs without much difficulty when the air is steady.
A tougher test of telescope resolving power is Porrima, Gamma (g) Virginis. The components are identical in both magnitude (3.5) and color, making them a perfectly matched pair. Porrima is easy to locate in the bowl of Virgo above Spica. This is a binary system with a period of 171 years. It reached its minimum separation of 0.4" in 2005.
If you find Porrima too challenging, try searching in Ophiuchus, about midway between Antares and the Teapot of Sagittarius. Here you'll find another perfectly matched pair in 36 Ophiuchi, whose magnitude 5.1 components are separated by 4.9".
Courtesy Rainer Anton.
Everyone's favorite colored double is Albireo, Beta (b) Cygni,
and I certainly love it too. But have you tried viewing it in 15x binoculars? If mounted or stabilized, they'll certainly resolve it. Other colorful beauties that deserve attention are the triple Omicron (o) Cygni
(also easy in binoculars), 24 Comae Berenices,
and Epsilon (e) Boötis.
If you have trouble resolving the latter, try a pale blue filter. This trick helps with other close colored pairs and on nights of marginal seeing I've even used it to split Antares.
Two lesser-known but very pretty colored doubles with generous separation reside in Hercules: Alpha a Herculis (Rasalgethi) and 95 Herculis. Rasalgethi is easy to locate in southern Hercules, just 5° north-northwest of Alpha Ophiuchi.
Winding its way near the north celestial pole, the constellation Draco is blessed with the Northern Hemisphere's two best binocular doubles.
Courtesy Rainer Anton.
They feature separations ideal for 10x viewing. Nu (n) Draconis
is a perfectly matched pair with components of magnitude 4.9. Nearby 17-16 Draconis
is nearly its equal.
Three other doubles are so easy to find that they rarely escape my telescope on summer evenings. These are Algieba (g Leonis), Cor Caroli (a Canum Venaticorum), and Acrab (b Scorpii).